The inventive concept relates generally to memory systems, memory devices, and methods of operating a memory system. More particularly, the inventive concept relates to methods of operating a memory system including a nonvolatile memory device and an erase operation.
Memory systems including one or more nonvolatile semiconductor memory devices have become staple components in contemporary consumer electronic products. A variety of nonvolatile semiconductor memory devices are known, including as examples, the electrically erasable programmable read only memory (EEPROM), the phase-change random access memory (PRAM), the magnetic random access memory (MRAM), and the resistance read only memory (ReRAM). Within the broad class of nonvolatile semiconductor memory devices, flash memory provides certain advantages, such as rapid reading speed, low power consumption, very dense data storage capacity, etc. As a result, many contemporary memory systems incorporated in contemporary digital computational platforms and consumer electronics include flash memory as a data storage medium.
A charge trap flash (called “CTF”) memory device may be applied to 3-dimensional structure flash memory (3D flash memory) to overcome a physical limit of high-integration. Such configurations tend to increase the per unit area, data-storage integration density of the constituent nonvolatile flash memory device.
However, CTF memory cells are not without their own challenges. In particular, CTF memory cells often suffer from a phenomenon referred to as “initial verify shift” or “IVS”. This phenomenon is characterized by an undesired rearrangement or recombination of charge carriers (electrons and/or holes) on a charging storage layer of CTF memory cells following execution of a program operation or an erase operation.
Such charge carrier rearrangement tends to shift the threshold voltage of the CTF memory cells in a manner that may lead to data loss. Most troubling, the IVS phenomenon happens over a period of time following the programming (or erasing) of memory cells, and as such, memory cells already verified as having been properly programmed (or erased) may experience a threshold voltage shift that essentially changes the programmed (or erased) state of the memory cell.